In order to study the fate of specific enzymes in vivo, we have examined the levels of oxidized proteins, specific enzyme inactivation and protease activity in isolated hepatocytes of rats exposed to 100% oxygen for 54 hours and of rats of various ages, 3 to 26 months. Our studies indicate that oxidized proteins accumulate during 48 hours of oxygen treatment. Over the same time interval, at least two enzymes (glucose-6-phosphate dehydrogenase and glutamine synthetase) exhibit a decrease in specific activity without a concomitant loss of immunological cross reactivity. Between 48 and 54 hours of oxygen treatment, the levels of oxidized proteins decrease and the levels of alkaline proteases which exhibit selectivity for the oxidized proteins increase. These results are consistent with the interpretation that oxidized proteins increase in vivo during exposure to high oxygen and are subsequently degraded, possibly by selective proteases which are induced or activated. Similar studies carried out with rats of various ages indicate that oxidized proteins and enzymes are inactivated during normal aging. In this model, however, there is little loss of immunological cross-reactivity; moreover, the level of alkaline proteases which selectively degrade oxidized proteins is lower in old animals compared to young animals. Similar parameters were examined in rat testis from the Lobund Aging Studies. In these experiments, oxidized proteins were compared in conventionally maintained and in germ-free animals that were either diet-restricted or full-fed.